I'm trying to devise a clever character that would be able to hide something quite large (say, a mountain, or a castle atop a tall tower) within the rules of the game. I am aware of spells such as mirage arcana and hallucinatory terrain, but as far as I know, they can't be made permanent, not to mention that it would require multiple castings to cover a castle.

What are some ways that a character could hide something large like this? I've listed Pathfinder as well as 3.5 and 3e because I expect that the answer will be very similar between the three.

7 Answers
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(Edited commentary): A nature of the D&D game is that illusion and detection spells are in an arms race to reduce the prevalance of scry & die tactics. From a tactical point of view, the resources spent hiding a fortification are either better spent hiding something smaller but more secret or fortifying the same structure against expected enemy tactics. The lowest item cost is approximately a hundred thousand gold, and that's for a custom item that the DM may not allow. The only time when concealing a fortification makes sense is during its magical (don't bother if you actually have to hire people to construct it) construction in a barren wasteland/desert as part of the first stage of defences against an enemy able and willing to use armies. Rendering a castle hard-to-detect takes more than visible illusions.

The most effective anti-detection methodology is to not be there in the first place. Enchanting the ability to planeshift/teleport/fly into the structure is a necessary pre-requisite to hiding the structure, as without that enemies can rely on fairly simplistic means of kinetic detection. or the spell "Find the Path." As most structure-hiding spells are at 5th-6th level, find the path must be factored in as a potential threat.

Once the structure is enchanted to move (random-walk if possible, which is why the best method is some sort of planar travel), the first layer of detection-defense must be enchanting false vision into every part of the structure. Again, without considering metamagic cheese, a tower of london structure has a volume of approximately 1136340 cubic feet. (Not counting outer fortifications). Every casting of false vision consumes 268083 cubic feet, so 5 castings will be necessary to cover the entire structure. This will protect against trivial scrying, but not find the path.

The probable line of attack, given that the structure is on a random plane, is to use contact other plane or its equivalent to have an outsider answer "Is $location on $plane?" Repeat for a number of castings. Said outsiders may or may not know the answer, but given that the caster is contacting notable powers on the plane and that the structure may impinge on their domain, there's no reason why they wouldn't know the answer. Still, this forces the enemy to use non-trivial resources. However, neither screen nor false vision will prevent find the path from working. Happily, find the path will not provide a useful teleport target and false vision is sufficient to block simple scrying sensors sent upon the path. (As a note, obsucre object could block find the path... if it could be scaled up to cover a location. It would be worth doing original research so that obscure object is a cubic spell, rather than an object spell. Barring that, as that's a house rule, detect scrying might detect the find the path sensor, but probably wouldn't. Nondetection functions upon "object touched" and certainly prevents divination... but not without a chance of failure. It's worth enchanting it into the castle at an extremely high level. Private sanctum fails due to Find the path not being a scrying subschool.

Fundamentally, the only real protection against find the path is a fairly frequent random planar walk of the castle combined with a high caster-level nondetetion and implemented "false vision" spells.

That being completed, the logic below is valid.

(End edit)

There are a number of options here, depending on your budget.

The simplest method is to simply use Mirage Arcana enchanted into an item. At base caster level (not thinking about cheesing items here, there are rather large resources devoted to that act) a level 5 spell requires a level 11 casterlevel.

This provides for a surface area of 20 feet * 11, or 220 feet that needs to be renewed every 11 hours.

Looking at a fairly trivial example of the tower of london, a nice square keep.

The White Tower is somewhat irregular in plan, for though it looks so square from the river its four sides are all of different lengths, and three of its corners are not right angles. The side towards which we approach is 107 feet from north to south. The south side measures 118 feet. It has four turrets at the corners, three of them square, the fourth, that on the north-east, being circular. From floor to battlements it is 90 feet in height. The original entrance was probably on the south side, and high above the ground, being reached as usual in Norman castles by an external stair which could be easily removed in time of danger. Another or the same entrance led from an upper storey of the palace. The interior is of the plainest and sternest character. Every consideration is postponed to that of obtaining the greatest strength and security. The outer walls vary in thickness from 15 feet in the lower to 11 in the upper storey. The whole building is crossed by one wall, which rises from base to summit and divides it into a large western and a smaller eastern portion. The eastern part is further subdivided by a wall which cuts off St. John's Chapel, its crypt, and its subcrypt, each roof of which is massively vaulted. There is no vaulting but a wooden floor between the storeys of the other part. There are several comparatively modern entrances.

So 110x90 for surface area, or 9900 square feet per side. A casting provides a surface area of 220 square feet. Therefore, with this el-cheapo "put the spell inna stick" method, costing (as per here) command word activated, 11*5*1800, or 99000 gold, It will take a minion roughly 5 minutes per side to fully renew the enchantment. It's reasonable to have 4 watches and to have the captain of the watch renew the enchantment every watch.

Another option is the use of a bunch of scrolls of Permanent Image. Best incorporated into surrounding terrain, you can simply disguise your castle as a hill. While you'll want skills in some sort of artistic craft, creating a bunch of permed images of "portions of a hill" should function adequately to your purpose. The cheapest option (besides from spending a non-trivial amount of time casting it yourself) is with a spell-trigger item. At a 6th level spell, it can create image covering a surface area of 150 feet. To cover the 5 sides of the tower with believable "hill" coverings, you need to cast it 330 times, which means it's probably just more effective to put it in a command word activated stick. This has the benefit of not needing regular re-casting.

However, we're just getting started.

With a little more imagination, it should be possible to build the castle out of invisible stone. One gets invisible stone through judicious use of the "wall of stone" spell and the Invisible spell metamagic:

Benefit: A hidden spell incorporates an illusion that renders it and its effects invisible. Invisible Spell does not conceal the casting of the spell, only the spell effects. An invisible spell uses up a spell slot three levels higher than the spell's actual level.

Notes: If the spell reasonably makes noise, has odor, or a tactile effect of some kind these effects are not hidden, it operates like any other invisibility effect. The “spell effects” refer to any magical manifestation of the spell, not indirect results of the spell. For instance, a ball of fire would be invisible but anything it ignites would be visibly burning.

As a sor-wis 5, spell, an invisible wall of stone is 8th level unless one employs typical tricks for reducing caster level of meta-magic. Still, this means that the fundamental infrastructure of the castle is invisible, reducing the problem to rendering the inhabitants invisible. A far more solvable problem.

Assuming one enchants a stick of permanency, a sor/wis 5 spell, at casterlevel 10 (it's more effective to do casterlevel 13 so you can make more things perminant) you can simply enchant a stick of invisiblity and render every stone and every item coming to the castle invisible, and grant all inhabitants a free see invisiblity.

A stick of permanancy costs... 5 gold per 1 XP on top of the base price, or roughly 340,000 gold for a stick at caster level 10. A worthwhile investment, however. The way to pay off the investement is to bring the minimum caster level to 12, so you can create endless walls of fire, for 608,000 gold and thereby dump functionally unlimited energy into the local civilization, with a concomitant increase in living standards.

There is, of course, the thoughtbottle option, but some things are too cheesy even to suggest.

All things being equal, the best way to hide a castle is to build it inside of the landscape and hide the bits that you changed with perm image. Beyond that, an item of mirage arcana and 4 shifts of guards should serve to hide the castle. The problem with this is that the castle is still vulnerable to scry&die and the most trivial examination of the local terrain (due to the natural layout of castles and their infrastructure (read: villages) ). While the infrastructure problem is also solvable, at a certain point it may make more sense for the castle to be flying (arms and equipment guide, probably, to which I don't have access right now, or the stronghold builder's guidebook).

Both of those books probably also detail additional building camouflage techniques, though of dubious utility.

Remember, when trying to hide a castle, you need to make sure it can hide against the expected threat. Make sure to perform a threat estimate before committing any gold, because most of these methodologies can be defeated by fairly trivial divination that a party of sufficient level to assault the castle will have access to.

For additional amusement, have your invisible walls be crafted by a shadowcrafter gnome such that the castle is only real if you believe in it. And/or the walls are hyper-real if you believe in them.
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Brian Ballsun-StantonJun 13 '11 at 3:01

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Never mind whether it appropriately participates in the magical arms race—hiding an entire castle is cool and the sort of thing that keeps fantasy fantasy.
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SevenSidedDie♦Jun 13 '11 at 13:56

It's even crazier if they're only hyper-real if you don't believe in them. Which is great for things like traps.
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chaosysJun 14 '11 at 14:57

Here is an engineer's suggestion: If you can accept the fact that is hidden only when observed from farther than, say, 500 meters, you may use an illusion not in the castle itself but in the space between the observer and the castle. I'm just relying on conical projection here, you can easily hide a fort behind your thumb if you're looking at it from that kind of distance, so I believe you could use magic easily as that.

To recommend a different RPG for a moment, Houses of Hermes: Societes from Ars Magica has an absolutely fantastic treatment of this technique. While it may not be suitable for wholesale importation into 3.5, it may give some interesting spells to invent.
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Brian Ballsun-StantonJun 13 '11 at 4:28

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This would work really well if the approaches are suitably limited.
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SevenSidedDie♦Jun 13 '11 at 9:07

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I gave it a second thought last night and it also seems a pretty good technique to make this illusionist of yours look powerful. For example, I got some pretty interesting responses from my players once when a changeling they believed to be a vampire was seen in daylight. :)
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rafgoncalvesJun 13 '11 at 12:08

Mirage Arcana is your best bet; of course it takes multiple castings, but it's a big thing you're trying to do. There's no good reason it can't be made permanent, they just nerfed that spell to a small list in the name of "game balance," as a GM I'd certainly allow it. Or you could make a higher level spell combining permanent image and mirage arcana.

Mage's Private Sanctum conceals the place and can be made permanent RAW; it's obvious that there's something there though.

The high level combo platter of all of these is Screen. Does vision and scrying, not permanent but lasts 24 hours, and a 30' cube per level would handle most manmade structures.

I'm with Brian on this one: He wouldn't (at least not by magical means entirely).

The only way to truly hide a castle is from the beginning otherwise it'll wind up on maps, in local memories, etc, etc.

A savvy illusionist should also be a master of other kinds of illusion, camouflage and so forth. A well constructed fortification can be masqued by building an insane spot/search DC into the very design itself. The best part about using mundane illusionism? Congratulations, you're immune to True Seeing! Consider using all manner of terrain to disguise the actual fortification and build in harmony with that terrain. Consider a significant portion of the castle being underground.

For those portions that absolutely must be visible you can use the suggestions Brian has, but remember that magically infused stuff is easily detectable by savvy searchers. The good news there is that you've dramatically reduced the amount of volume/mass/etc that must be baffled.

TL;DR For grand-scale stuff you want to use as much of a mundane approach as you can to reduce how much magic you need to use.

I'm sure this is way too late to be of any use for you, but I'd be tempted to try the permanent image approach, with 'shape spell' metamagic in order to spread the cube out over a much larger surface.

I'd treat only the outside walls with it, and instead of making the castle 'disappear' (or appear as something else), I'd make it appear totally ruined (possibly burnt) and uninhabited. I'd also make the ramparts appear 5ft higher than they really are, in order to hide defenders behind them more easily (they'd know about the illusion and could be trained to ignore it)

The first measures of defense could be to concentrate on parts of the wall to make them appear to crumble, or show animals that don't live near inhabited places (a good idea if the players have a high int (bard or wizard) type who always tries to 'bardic knowledge' everything ;)

If it fits with the situation you need it for, you could just have a single entrance to the place you're concealing (a forest path or a mountain pass) and place some sort of deceptive veil in it.

Instead of using magic on the place you want to hide, you could use magic on the people you're hiding it from. Maybe lowering the sight range of anyone who comes near the place so they can't see it until they're up close? (Of course, this provides its own set of problems)

If you really want to hide the entire thing and are worried about large spells, you could place a handful of small devices/magic orbs/etc. that are specifically crafted and synchronized to constantly pulse out this arcane field that conceals your area. Maybe finding the area involves your players stumbling upon one of these hidden devices and figuring out how to turn it off.

Several Permanent image spells to completely cover it on all sides to appear to be a mountain...then imbue every brick in castle with non-detection (helps to enchant the mortar during construction as it's much more cost effective then doing it afterward).

Or you could make an epic spell (if you're that high) that does basically the same thing. Delude, ward and conceal seeds, permanent duration, several range increases for the conceal and ward effects, uses a ritual with...a bunch of hired npc casters, to assist in the casting ritual to bring the DC down to something reasonable...then when it's finished kill them all so they can't tell anyone else about it, or use a epic memory modification to wipe out the the entire event (and get out of paying them).

Spell cloaks the entire castle grounds in an illusion of a mountain while misdirecting divinations to report that it's simply a mountain, also blocks spells that detect magic.